THERMAL TRANSFER ELEMENT STRUCTURE BACKGROUND OF THE INVENTION The present invention relates to structures of thermal transfer elements and more specifically to a structure of heat absorbing plates to use * ert a heat exchanger, where the heat is transferred through the plates from a hot thermo exchange fluid to a cold thermo exchange fluid. More particularly, the present invention relates to a thermo exchange element structure, adapted for use in a thermal transfer apparatus of the rotary regenerative type, wherein the thermo transfer element structures are heated by contact with the thermo exchange fluid hot gaseous and subsequently put in contact with cold gaseous heat exchange fluid to which the thermal transfer element structures, yield their heat. One type of thermal transfer apparatus to which the present invention has particular application is the well-known rotary regenerative thermo exchanger. A typical rotary regenerative heat exchanger has a cylindrical rotor divided into compartments where spaced thermal transfer plates are placed and held, which, as the rotor rotates, are alternately exposed to a hot gas stream and then to rotor rotation, a stream of cold air or other gaseous fluid to be heated. As the heat transfer plates are exposed to the heated gas, they absorb heat therefrom and then, when exposed to cold air or other gaseous fluid to be heated, the heat absorbed from the gas heated by the heat transfer plates is transferred to the gas. cold. Most of the thermos
exchangers of this type have their heat transfer plates stacked closely in spaced relation, to provide a plurality of passages between adjacent plates for the flow of thermo exchange fluids therebetween. This requires means associated with the plates to maintain adequate spacing. The thermal transfer capacity of this thermo exchanger of a given size is a function of the thermal transfer rate between the thermo exchange fluids and the plate structure. However for commercial devices, the utility of a device is determined not only by the obtained thermal transfer coefficient, but also by other factors such as the cost and weight of the plate structure. Ideally, thermal transfer plates will induce a highly turbulent flow through the passages between them in order to increase thermal transfer from the heat exchange fluid to the plates, while at the same time providing a relatively low resistance to flow through of the passages and also have a surface configuration that is easily cleaned. To clean the thermal transfer plates, it has been usual to provide soot blowers, which supply a burst of air or steam with high pressure through the passages between the stacked thermal transfer plates, to release any deposits of particles from their surface and drag them leaving a relatively clean surface. This also requires that the plates be properly separated to allow the blowing medium to penetrate the stack of the plates.
One method of maintaining plate spacing is to fold the individual heat transfer plates at frequent intervals to provide notches extending away from the plane of the plates to separate the adjacent plates. This is often done with two-lobed notches, which have a lobe that extends away from the plate in one direction and the other lobe that extends away from the plate in the opposite direction. Structures of thermal transfer elements of this type are described in U.S. Pat. Nos. 4, 396,058 and 4,744,410. In the patent, the notches extend in the direction of the fluid flow of general heat exchange or mass, that is, axially through the rotor. In addition to the notches, the plates are corrugated to provide a series of oblique grooves or grooves extending between the grooves at an acute angle to the fluid flow of heat exchange. The corrugations in adjacent plates extend obliquely to the mass flow line, either in an aligned manner or opposite each other. These undulations tend to produce a highly turbulent flow. Although these thermo transfer element structures exhibit favorable heat transfer rates, the presence of straight extending notches through the direction of mass flow provides significant flow channels that bypass or short circuit the fluid around them. major undulating areas of the plates. There is a higher flow expense through the notch areas and a lower flow expense in the corrugated areas that tend to reduce the heat transfer rate. SUMMARY OF THE INVENTION An object of the present invention is to provide an improved thermal transfer element structure, wherein the thermal performance is optimized to provide an improved level of thermal transfer. A desired plate spacing and a reduced amount of plate material. According to the invention, the thermal transfer plates of the thermal transfer element structure have oblique corrugations to increase turbulence and thermal performance, but they do not have straight notches that extend axially for plate spacing. On the contrary, at least each sautéed plate
contains portions or depressions locally raised, of a height to adequately separate the plates. The depressions are formed by stretching or pulling the material, reducing locally the amount of plate compared with the notched plates. The corrugations in adjacent plates may extend in opposite directions to each other and the direction of fluid flow. Brief Description of the Drawings Figure 1 is a perspective view of a conventional rotary regenerative air preheater, containing structures of thermal transfer elements constituted by thermal transfer plates. Figure 2 is a perspective view of a structure of
conventional thermal transfer element showing the thermal transfer plates stacked in the structure. Figure 3 is a perspective view of portions of three stacked thermal transfer plates for a thermal transfer element structure according to the present invention, illustrating the
undulations and spacing depressions.
s ^^. ^^ - a - * «• *. - Figure 4 is a cross section of a portion of one of the plates of Figure 3, which illustrates the corrugations or depressions. Figures 5 and 6 are illustrations of two of the various depression configurations. Figure 7 is a cross-section of portions of three plates of a stack showing a variation of the invention. Figure 8 illustrates a roller forming method for producing depressions with a roller to adjust varying plate lengths. DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to Figure 1 of the drawings, a conventional rotary regenerative preheater is generally designated by the numerical identifier 10. The air preheater 10 has a rotor 12, rotatably mounted in a housing 14. The rotor 12 It is formed of diaphragms or separations 16, which extend radially from a rotor post 18 to the outer periphery of the rotor 12. The separations 16 define compartments 17 therebetween to contain structures of thermo exchange elements 40. The housing 14 defines a duct of combustion gas inlet 20 and a combustion gas outlet duct 22 for the flow of heated combustion gases through the air pre-heater 10. The housing 14 further defines an air inlet duct 24 and a duct outlet air 26 for the flow of combustion air through the preheater 10. Sector plates 18 extend through the housing 14, adjacent s to the upper and lower faces of the rotor 12. The plates of the sector 28 divide the air pre-heater 10 into an air sector and a combustion gas sector. The arrows in Figure 1 indicate the direction of a flue gas stream 36 and an air stream 38 through the rotor 12. The hot combustion gas stream 36 accesses through the flue gas inlet duct 20, transfers heat to the heat transfer member structures 40 mounted in the compartments 17. The structures of the heated transfer element 40 are then turned to the air sector of the air pre-heater 10. The heated air of the transfer element structures 40 is then transferred to the combustion air stream 38 by accessing through the air inlet duct 24. The cold combustion gas stream 36 exits the preheater 10 through the combustion outlet duct 22 and the hot air 38 leaves pre-heater 10 through the air outlet duct 26. Figure 2 illustrates a basket or typical thermal transfer element structure. ca 40, which shows a general representation of thermal transfer plates 42 stacked in the structure. Figure 3 illustrates an embodiment of the invention showing portions of three stacked thermal transfer plates 44, 46 and 48. The direction of mass fluid flow through the plate stack, indicated by arrow 50. The plates are thin sheet metal capable of being rolled or die cut to the desired configuration. The plates each have corrugations or corrugations 52 extending at an angle to the direction of fluid flow. These undulations produce turbulence and improve thermal transfer. In the preferred embodiment as illustrated in this Figure 3, the corrugations in adjacent plates extend in opposite directions to each other and the direction of fluid flow. However, the corrugations in adjacent plates may be in the same direction parallel to each other. Although the corrugations shown in Figures 3 and 4 are continuous with one undulation leading directly to the next, the corrugations may be spaced apart with flat sections between two corrugations. The two plates 44 and 48 which are identical to each other have depressions 54 and 56 formed thereon for the purpose of spacing adjacent plates. The depressions 54 extend upwardly and the depressions 56 extend downwardly in this Figure 3 and as illustrated in Figure 4, which is a cross section of a portion of the plate 44, through two of the depressions. The height of these depressions 54, 56 is greater than the height of the corrugations 52, as seen in Figure 4. The depressions are narrow and elongate in the direction of fluid flow. The narrow width dimension minimizes undesirable pressure flow and pressure drop. The elongated length provides the necessary support, always leaning on at least one of the corrugations. Therefore, the minimum depression length is at least equal to the pitch of the corrugations and preferably longer to allow manufacturing tolerances. However, if the depressions are very long, the flow will begin to channel, without interacting with the adjacent undulations. Therefore, the depressions should not be longer or more frequent than those required for adequate spacing and for structural support to withstand soot blowing and high pressure water washing. In general, the total accumulated depression length in a row in the direction of flow should be less than 50% of the plate length. Preferably, this length of
Total depressions should be 20 to 30% of the plate length. As an example, the length of depressions can be 3,175 cm. (1.25 in) with spacings between depressions of 8.89 cm. (3.5 ¡n). The pattern of depressions may vary as desired. For example, the pattern may be alternating rows in line of up and down depressions, which alternate between adjacent rows in the longitudinal direction of flow 50, as shown in Figure 5 alternating between adjacent transverse rows or adjacent diagonal rows. In another example, the depressions may be arranged in a diamond pattern, as illustrated in Figure 6. Again, the alternate rows may be longitudinal, transverse or diagonal. As indicated, the embodiment of the invention of Figure 3 has only depressions in each sautéed plate, which is all that is necessary for purposes of spacing with the up-down pattern of depressions. However, the depressions can be located on each plate and the depressions on each plate can be on one side of the plates. Figure 7 shows a cross section of portions of three stacked plates 58, having the corrugations 52 but each having depressions 60 extending all the way to the same side of the plate. The depressions are formed by a roll forming or press forming process, which locally stretches and deforms the metal. The preferred method is roll forming due to the inherent faster production speed. This is contrasted with the formation of the notches in the prior art, which is a bending process without stretching or significant deformation that consumes material and requires a wider metal sheet. He
Stretching process that deforms and stretches the metal, does not consume material. The material savings are approximately 8%. In the present invention, it is preferred that the depressions at one end or probably both ends of the plate are relatively close to both ends for the purpose of reinforcing and supporting the ends of the plates. This is particularly convenient at the ends of the plates subject to frequent and / or washing with water or blowing soot with high pressure. The depressions at these ends prevent or reduce plate deflection and fatigue and improve the life of the plate. One selection is to make the depressions are close and spaced only slightly from the ends, probably 1,905 cm. (3/4 in) approximately or less. The other selection is to make the depressions actually extend to the extremes. One way of forming plates with the depressions extending to the ends and allowing the formation of plates of varying lengths is illustrated in Figure 8. This is a plan view of a forming roll 60 containing a pattern of depressions and a portion of a plate 62 formed. A complementary forming roller will be located below the roller 60 and the plate passes between the two forming rollers. The forming rolls are long enough to allow plates of the maximum expected length and have a pattern of depressions to also allow or adjust to shorter plates. At the ends (or at least one end) of the roll 60 are depressing patterns 64 having an extended length greater than the length of a desired normal depression. The patterns of formation of depressions 66 between the ends are of normal length. As an example, the patterns of formation of depressions 64 may be approximately 10.16 cm (4 ¡n) in length, while the patterns of formation of normal Depressions may be approximately 3.175 cm (1.25 in) previously mentioned. This roller 60 can in this way accommodate a plate as long as "A" or as short as approximately "B" and still have depressions formed in both ends of the plates. The present invention provides material savings and improves heat transfer. Also, the plate assembly is open to allow easy cleaning by soot blowing or washing with water, to remove scale deposits and allow the escape of infrared radiation for the detection of excessive temperature conditions.